Something you do to make yourself seem cooler than you are. A lot of the time you don't really realize you're doing it, but all of the time you look a fool to everyone watching, assuming they've grown up more than you have.

Almost everyone has some affectations, but people with a lot of them stick out like sore thumbs and they don't even know it. See try-hard.

People in high school and college (especially) tend to have a lot of affectations because they are stupid. Or because they don't have the life experience to know that they are being stupid. Oftentimes both.

The college sophomore down the hall who wears a fedora every day (known, unknowingly, to his hallmates as "Fedora Guy"): "I like to think I'm an endearingly mysterious gentleman."

The kid with the fake accent/other contrived manner of speaking: "The way I casually throw in SAT words into my everyday speech makes the Normals uncomfortable, which is really cool because that means I'm impressing them and earning their respect."

Self-proclaimed nerd: "I really like showing people that I know a whole lot about really obscure things like math and the internet, because when I meet people who are 'in the know' like me and who actually deserve to be in my presence, they'll know what I'm talking about and they'll be really impressed, which is epic hacks!"

Dumb college kid at top-tier liberal arts school: "I'm so glad I've finally found a community where people like me can just bounce ideas off one another. I'm really excited about completely changing the way I think about everything by surrounding myself with such a diverse population of people who are exactly like me. Also I really really care about the actually important stuff, like politics and Darfur and voting and political correctness and intellectual conversations and getting drunk once in a while and being offended while remaining open-minded."

Generic affected individual: "What? No, that's not an affectation, that's just the way I've always done that. No, you've got it all wrong--I do it for a reason. Plus--heh heh--it just plain looks cool."